1. Routing and Routing Protocols
CCNA 2 – Chapter 6

2. Routing Types

3. Static Route Operation
Hoboken#show ip route
Codes: C - connected, S - static,
S /24 [1/0] is directly connected, Serial0
C /24 is directly connected, Ethernet0

4. ip route command
RTR(config)# ip route prefix mask {address | interface} [distance]
prefix IP route prefix for the destination.
mask Prefix mask for the destination.
address IP address of the “next hop” that can be used to reach that network.
interface Network interface to use (exit-interface)
distance (Optional) An administrative distance.

5. Static Route Operation
If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed in the routing table.
This means if that interface is down, the route will not be placed in the routing table.
If the exit interface (gateway) is “down” the static route will not be put in the routing table.

6. Administrative Distance and Metric
Hoboken#show ip route
Codes: C - connected, S - static,
S /24 [1/0] is directly connected, Serial0
C /24 is directly connected, Ethernet0
[ administrative distance / routing metric (or cost) ]
The cost for all static routes is “0”
The default administrative distance for static routes is “1”

7. Administrative Distance
Administrative Distance is the “trustworthiness” of the routing information.
Lower the administrative distance the more trustworthy the information.
If the router hears about a route to the same network from more than one source it will use the administrative distance to decide which route to put in the routing table.

8. Examples

9. Examples
The network and mask are known as a “default route”
Can be written /0
Known as a “quad zero” route”

10. Static Routing Configuring static routes
Routers do not need to configure static routes for their own directly connected networks.
We need to configure static routes for networks this router needs to reach.
We will need to configure static routes for the other routers as well, as “routing information about a path from one network to another does not provide routing information about the reverse, or return path.”
Convergence – When all the routers in the network (AS) have accurate and consistent information, so that proper routing and packet forwarding can take place.
Convergence will not happen until all the routers have complete and accurate routing information, meaning we must configure static routes on all the routers before packets will be correctly delivered.

11. Static Routing 2 1 Recursive Lookup
RTA(config)#ip route
RTA#show ip route
Codes: C - connected, S - static,
C /16 is directly connected, Serial0
S /24 [1/0] via
C /24 is directly connected, Ethernet0 2 1
It does a recursive lookup – first (1) by looking up the /24 network and finding it needs to forward the packet to – the router then (2) looks up the network and sees it can forward it out the interface Serial 0.
Recursive Lookup
The router knows it can get to /24 network by forwarding the packets to the router at the ip address of
How does the router know how to get to the ip address ?

12. Common uses for Static Routes
/24
ISP
ip route
/24
ABC
/16
Static routes in the real-world
In the example above, there is only one route, link, between ABC’s network and the ISP.
When there is only a single route to a network, this is known as a stub network.
It is very common for the ISP to have a static route pointing to it’s customers’ networks, in this case ABC.

13. Common uses for Static Routes
Default
/24
ISP
ip route
/24
ABC
ip route
/16
Static routes in the real-world (continued)
What about ABC and sending packets to the ISP – packets going to the Internet?
It is also common for customer networks to use a special kind of static route, known as a default static route.
Of course we will examine this later throughout the rest of this course, but for now we specify the network and mask as “ ” (pronounced “quad-zero”).
This tells the router to forward all packets to this next-hop address (or exit interface) that do not have an explicit route in the routing table.

14. Common uses for Static Routes
Default
/24
ISP
ip route
/24
ABC
ip route
/16
RTB#show ip route
Gateway of last resort is to network
C /16 is directly connected, Ethernet0
/24 is subnetted, 1 subnets
C is directly connected, Serial1
S* /0 [1/0] via
Any packets not matching the routes /16 or /24 are sent to the router – where it is now their “problem.”

15. Summarizing static routes
There are many times when a single static route can replace several static routes.
In other words, summarizing several static routes into a single static route.
/24

16. Summarizing static routes
Baypointe
The three static routes can be summarized into a single route:
Baypointe(config)# ip route
Baypointe(config)# ip route
   Baypointe(config)# ip route
Summarized route:
Baypointe(config)# ip route
The summarized route will now include all three subnets!
Be sure to use the proper mask – !
Using a mask will only route for /24 subnet and not /24, /24 or /24.

17. Summarizing static routes
/24
Baypointe
Summarized route:
  Baypointe(config)# ip route
Advantages:
Fewer routes in the routing table – faster routing table lookup.
Subnets can be added and deleted on network without having to change static route on Baypointe router.

18. Verify static routes
Copy running-config startup-config

19. Dynamic Routing Protocols

20. Routed Protocols vs. Routing Protocols

21. Autonomous Systems
An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy.
To the outside world, an AS is viewed as a single entity. The AS may be run by one or more operators while presenting a consistent view of routing to the external world.
The American Registry of Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying 16-bit number to each AS.

22. Routing Protocols
The goal of a routing protocol is to build and maintain the routing table.
This table contains the learned networks and associated ports for those networks.
Routers use routing protocols to manage information received from other routers, information learned from the configuration of its own interfaces, along with manually configured routes.

23. Types of Routing Protocols
Distance Vector: RIP, IGRP, EIGRP
Link State: OSPF, IS-IS
Path Vector: BGP
Note: IGRP and EIGRP are Cisco Proprietary

24. Distance Vector Routing Protocols
Router B receives information from Router A.
Router B adds a distance vector number (such as a number of hops), which increases the distance vector.
Then Router B passes this new routing table to its other neighbor, Router C.
This same step-by-step process occurs in all directions between neighbor routers.
“Routing by rumor”
Each router receives a routing table from its directly connected neighbor routers.

25. Distance Vector Routing Protocols

26. Distance Vector Routing Protocols
Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router.
With some routing protocols routing tables updates happen on a periodic basis.

27. Link State Routing Protocol Operations
Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers.
Topological database – A topological database is a collection of information gathered from LSAs.
SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree.
Routing tables – A list of the known paths and interfaces.

28. Link State Routing Protocol Operations

29. Path Determination
A router determines the path of a packet from one data link to another, using two basic functions:
A path determination function
A switching function

30. Path Determination
The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router.
A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.

31. Configuring Dynamic Routing

32. Configuring Dynamic Routing
GAD(config)#router rip
GAD(config-router)#network

33. Network command two things:
Router(config)#router rip
Router(config-router)#network
Router(config-router)#network
The network command is used on only directly connected networks.
With RIP and IGRP, only need to use the classful address (no subnets).
Network command two things:
Tells the router which interfaces that will participate in this dynamic routing protocol, which interfaces it will send and receive routing updates on.
Tells other routers the networks in its routing updates that it is directly connect to.

34. Distance Vector vs. Link State
Requests routing information from directly connected neighbors
Slower Convergence
Decisions based upon information provided by neighbors  
Flood routing information to all routers
Event-triggered updates, so convergence is fast
Complete view of the internetwork topology